I was just about to start a post about splitting Lightroom catalogues, but seeing as this is here already...
I simply organise my photos in folders by date on my hard drive as they are imported by Lightroom. I basically let Lightroom sort it out much as the respondents above.
I then use Lightroom collections and collection sets to put my photos into context, along with keywords. It really is essential to keyword your photos on import and during your post-processing work otherwise you'll never find the buggers.
I can therefore find my photos in two ways - by checking in my collections (i.e. I'll have a collection for each event, each sports match etc, and also collections for genres like urban, monochrome etc), and also by keyword.
Today I split my catalogue into 3. It was up to 15,000 images and things were starting to run slow, especially Develop. Splitting the catalogue was pretty easy. I decided to put all my Sports stuff in one catalogue, all my Events into another one, and left everything else in the original catalogue. Generally I'll know if I am looking for a Sports image, an Events image, or something else, so I'm not overly worried about not being able to find pics across the 3 catalogues.
To split the catalogue, I selected all the photos in the relevant collection set and did "export to catalogue" (or "catalog") and off they went. The new catalogues have all the collections and collection sets transferred over with no problem. Once I was happy all the photos were there, I deleted them out of the original catalogue (note - "Removed photos from Lightroom" rather than deleting them from disk!!!). Everything is nice & fast as a result. What a relief.
If I need to, I'll then start archiving off older photos to separate catalogues e.g. "Events 2007-8" etc.
Now, a nice little tip for you. To switch catalogues in Lightroom, Lightroom has to close and then re-open. This is a pain. So what I have done is associate the .lrcat filenames of the 3 catalogues with Lightroom 2.3. I then put shortcuts to those .lrcat files on my taskbar. I can now open a catalogue just by clicking on a shortcut. If Lightroom is already open in another catalogue, it closes and then re-opens the requested catalogue. All much easier than clicking menus and stuff.
Tobers